This invention relates generally to sealing bars for heat sealing a length of heat-sealable material.
This invention has particular (albeit not exclusive) application to sealing bars for form-fill-seal packaging apparatus. The majority of form-fill-seal packaging machines form tubing having a longitudinal back seam, such as an overlap seam or a "fin seam". The tubing is then flattened and heat seals are formed transversely to the axis of the tubing (and transversely to the longitudinal seam) by heat sealing bars to form packages. However, the transverse seals are very difficult to form at high speed without imperfections, such as weak or inconsistant seals. One cause of these imperfections is believed to be variations in the thickness of tubing (e.g., variations in thickness caused by gussets or seams).
For example, flattened tubing having an overlap seal has an increased thickness portion roughly 50 percent thicker than the flattened tubing generally (i.e., two layers of material at the seam plus one layer of material at the opposite side of the tubing is 50 percent thicker than the two layers of material generally found at opposite sides of the flattened tubing). In the case of a fin seal, the increased thickness portion is roughly twice as thick as the flattened tubing generally (i.e., two layers of material at the seam plus one layer adjacent the seam plus one layer at the opposite side of the tubing is twice as thick as two layers of material).
Since the tubing is of varied thickness, conventional sealing bars may not apply even pressure to the flattened tubing. Consequently, the pressure applied is likely to be too high at the thick seam portion, thus forcing all or most of the sealant away from the seam, and too low adjacent the seam portion, which may result in insufficient heating of the tubing adjacent the seam. In addition, the extra thickness of the seam may cause the sealing bars to bow out from the tubing, which may contribute to the problem since intimate contact of tubing material is important for heat sealing. These problems are particularly acute with some of the modern film materials, such as saran coated polypropylene, which may have a sealant coating which is only 0.10 mil thick.
Several attempts have been made to solve these problems. Probably one of the more common approaches has been to apply a flexible heat resistant material, such as silicone rubber, to the sealing bars to accommodate seams and gussets in the tubing. Rubber, however, is not heat conducting, and therefore it reduces the rate of heat transfer to the tubing and it takes longer to heat up to operating temperature. In addition, it has been found to be difficult to machine or mold and match the desired die tooth configuration into the rubber.
Another approach has been to cut away a portion of the heat sealing surface of the sealing bar to provide a relief for a longitudinal seam. One problem with this approach is that the longitudinal seam may "wander from side to side" and the overlap may not always be the same size, and thus this approach has not been found to be practicable. Yet another approach, which has been commercially applied, has been to precision grind teeth into the sealing surface, instead of machining the surface with a milling cutter. This approach reduces the severity of the sealing problems but does not eliminate them.